TypeScript Notes/Tips/Patterns

  • Scope: by default members in a class are public

  • Enum: an enum is a way of giving more friendly names to sets of numeric values

  • Do we put return type when defining function? Yes. getSum(a: number, b: number): number {}

  • Type assertion: (<string>someValue).length;

  • We have static, readonly

    • const is for variable. No one can modify it after assigned.

    • readonly is for property (class property to property in interface). Properties must be initialized at their declaration or in the constructor. It only ensures that it won't be modified by me. Others can still modify it. See here

    • static must be class member.

  • Function overload, this is possible in TypeScript but it's not supported by JavaScript.

  • Default parameter: getSum(data: number[], skipNegative: boolean = false)

Favor getter over method in TS

Type: How to declare type to object property and key?

var stuff: { [key: string]: string; } = {};
stuff['a'] = ''; // ok
stuff['a'] = 4; // error

Type: type of array item based on index

  • public label(row: {0: string, 1: number}): string {

  • var t:[number, string] = [1, "message"]

Use Destructuring (FP concept)

// pick only the data we need using destructing
// Function signature: getAccountInfo(): {balance: number; cardStatus: string; cardNumber: string; cardHolder: string}
// before:
.then((response:any) => {
const data = this.getAccountInfo()
return {
balance: data.balance,
cardStatus: data.cardStatus
// after
.then((response:any) => {
const {balance, cardStatus} = this.getAccountInfo()
return {

Be careful: it's too easy to create private member

If the data is NOT used anywhere, don't put private.

constructor(private data?: any) {
this.model = this.extract(data);